1. Field of the Invention
The present invention relates to an illuminated electrical part and, more particularly, to the improvement of an illuminating device for illuminating the indicator panel of an illuminated electrical part from behind the same.
2. Description of the Related Art
An illuminating device for illuminating the indicator panel of an electrical part from behind the indicator panel has been used widely, for example, for illuminating the indicator panel of an automotive electrical part during the night or while the automobile is in a tunnel. The illuminating device for such a purpose is desired to be capable of substantially uniformly illuminating the entire area of the indicator panel of an electrical part and of avoiding producing local, excessively bright or excessively dark portions in the indicator panel.
Referring to FIGS. 3 and 4 showing a conventional illuminated electrical part, the illuminated electrical part has a case 1 having an opening 2, and an indicator panel 3 fixedly covering the opening 2. A lamp 5, i.e., a light source, is fixed to a support plate 4 attached to the case 1 on the backside of the indicator panel 3, and a light conductor 6 formed of an acrylic resin is attached to the support plate 4 so as to cover the lamp 5. Light emitted by the lamp 5 is conducted by the light conductor 6 so as to illuminate the indicator panel 3 from behind the same. The illuminated electrical part is provided with through holes 7 for receiving operating members therethrough.
The light conductor 6 of the conventional illuminated electrical part, however, has errors in geometry, and hence the indicator panel 3 cannot be illuminated in satisfactorily uniform illuminance by the light emitted by the lamp 5. To compensate the irregular illumination of the indicator panel 3 by the lamp 5, color films are printed in layers over the indicator panel 3, in which a film of a dark color is printed over areas in which the illuminance is higher than other areas and a film of a bright color, which is more transmissive than the film of the dark color, is printed over areas in which the illuminance is lower than other areas so that the overall illuminance of the indicator panel 3 is uniform. Such a measure to compensate the irregular illumination of the indicator panel 3 increases the manufacturing cost of the electrical part and requires very difficult work to print the color films so that boundaries between the color films are not conspicuous.
Since the distance l (FIG. 3) between the lamp 5 and the light conductor 6 must be sufficiently large to obviate the adverse influence of heat generated by the lamp 5 on the light conductor 6, and the light conductor 6 must have a sufficiently large thickness, it has been difficult to form the illuminated electrical part employing such an illuminating device in a reduced thickness.